The present invention is directed to a safety device which prevents a gripper bar from being squeezed or engaged in a platen press that is processing plate-like workpieces, such as sheets of paper or cardboard.
A processing machine for sheets of paper or cardboard usually comprises an infeed station in which a pile of sheets is arranged. Every sheet is successively taken from the top of the pile in order to be carried onto a feeding table. On this table, every sheet is aligned on front lays or stops and side marks or stops prior to being seized at its front edge by a series of grippers mounted along a crossbar, whose ends are attached to a train of lateral continuous chains which carry the bar and, thus, the sheet into and through the processing stations. The processing stations can be a die-cutting platen press followed by a waste stripping station. The processing stations are usually followed by a delivery station in which every sheet is released by the grippers and aligned when dropped on a top of a pile which accumulates on an outlet pallet.
A platen press used in such a machine normally comprises a lower beam raised at each cycle of the machine until its upper plane surface will apply a pressure on a plane metal plate belonging to a fixed upper beam, which is mounted between the sides of the frames of the station. The movable beam is usually raised at four points, each by a toggle driven by a push-rod connected to the same crankshaft. This crankshaft is, in turn, driven or rotated by a vertical toothed gear which is engaged by an endless screw arranged on a horizontally rotating axis connected at one of its ends to an inertia flywheel driven by a motor, for instance, by means of belts.
As may easily be gathered, a machine cycle corresponds to a complete circuit of the crankshaft involving an ascent and descent of the lower beam. It is then handy to qualify the instants and the terms of this cycle into "machine angle" .theta. ranged between 0.degree. and 360.degree. in order to establish relationships with the successive positions of each element of the press and/or of the gripper bar in a way independent from the final production speed. For instance, the high dead center .theta..sub.PMH and the low dead center .theta..sub.PMB of the lower beam corresponds, respectively, to the angles of 50.degree. and 230.degree. of the crankshaft, whereas the outlet angle .theta..sub.sor when the gripper bar leaves the platen press corresponds to an angle of 298.degree. and a little before its standstill or dwell in the advancement of the gripper bars for positioning the sheets in the platen press during the die-cutting operation.
In the course of a run, it may happen that a sheet is not properly seized by the grippers or that two sheets are simultaneously taken or that one of them breaks, which fact, at the end, always causes an accumulation of sheets, also called a jam, in one of the stations of the machine. The occurrence of a jam converts immediately into an overpressure on the conveying chains for the gripper bars. A safety coupling arranged in the drive of this chain conveyor opens quickly and causes a standstill of the chains and then of the machine.
If a jam occurs when the gripper bar enters in the press, whereas the lower beam is close to its low dead center, a general standstill or stopping of the machine is sufficiently quick to stop the lower beam from rising. However, if such a jam occurs when the bar is just on the way to leave the press, the lower beam is being raised toward its high dead center position, the gripper bar might still stop in the press with no possibility of stopping the lower beam, which then squeezes or crushes the bar on the upper beam. Prior to starting the machine again, it is then necessary to change this damaged bar, which action extends the down-time of the machine.
In order to avoid this type of accident, a particular safety device is installed in the platen press to urgently and quickly brake this lower beam. The device used at present comprises a pick-up or detector in the safety coupling of the drive for the chain conveyor which detects an emergency opening. This pick-up then transmits the signal to a switching device which sets into operation a brake arranged in the drive of the crankshaft of the press, for instance jaws which will close either on the shaft of the endless screw, on the inertia flywheel or on a disc specially fitted parallel to the flywheel.
While this arrangement is satisfactory with certain presses, this safety device becomes inoperative for other presses as soon as the configuration or the track of the lower beam or of the drive elements or of the braking elements are slightly modified.